The present invention is an antimicrobial agent containing a polyalkyleneimine derivative formed by adding a substituent group having a structure of a following Formula (1) to a nitrogen atom of polyalkyleneimine. —CH2CH(OH)CH2—O—R1  (1) In the formula, R1 represents an alkyl group with 6 to 20 carbon atoms, an alkenyl group with 6 to 20 carbon atoms, an aryl group with 6 to 20 carbon atoms, or —(CH2CH2O)n—R2. R2 represents an alkyl group with 6 to 20 carbon atoms, an alkenyl group with 6 to 20 carbon atoms, or an aryl group with 6 to 20 carbon atoms. n represents an integer of 1 to 50.